Commercial broiler production evolved over the last several decades into an industry which is characterised by its high efficiency in the confinement rearing of the livestock. However, the strong tendency towards increasing of the efficiency of the rearing phase is not accomplished without encountering some inherent difficulties. Most notable is the increased incidence of infectious diseases that often occurs in high density, close confined animal populations. Many of the most devastating diseases in poultry have been limited or controlled by vaccination or by treatment with therapeutic agents such as antibiotics. Unfortunately, however, there are still a number of diseases of complex aetiology that have not been controlled with drugs and for which a suitable vaccination program is unavailable.
Since the late 1970s, the poultry industry has been confronted with such complex diseases in broiler chickens suffering from enteric problems. One of such diseases which results in a variety of disease conditions in affected chickens, including enteritis, is named for the main clinical sign and macroscopical observation: malabsorption syndrome (MAS). Alternatively, this disease is designated as infectious runting stunting syndrome, pale bird disease or brittle bone disease. Although a large number of assorted disease conditions are linked with MAS, in all cases poor growth and retarded feathering are observed. Additionally, a large variety of other signs and lesions, such as mortality, secretion of too liquid faeces and/or maldigested feed, pancreatic atrophy, proventriculitis, bone changes, thymic and bursal atrophy, etc. have been correlated with MAS.
Kouwenhoven et al. (Avian Pathology, 17, 879-892, 1988) defined MAS by five criteria, i.e.
(i) growth impairment up to 3 weeks after infection of one-day-old chicks
(ii) excretion of yellow orange mucoid to wet droppings
(iii) increased plasma alkaline phosphatase (ALP) activity
(iv) decreased plasma carotenoid concentration (PCC)
(v) macroscopically widened epiphyseal growth plates of the proximal tibia.
Retarded growth in broilers becomes obvious by 1 week of age or earlier. From 5 to 20% of the birds in a flock may be affected and these birds will be half the size or less of their penmates by 4 weeks of age. Affected flocks have poor feed conversions and the intestines are pale and contain undigested feed.
Although the pathogenesis of MAS is poorly understood, the likely pathogenesis of this syndrome is the direct action of infectious agent(s) on the digestive tract and associated organs, which would also explain the recurring secretion of too liquid faeces and/or maldigested feed.
The syndrome described here results in a general lack of performance, including diminished weight gains, poor feed conversion and reduced marketability of the affected flocks. As a result of MAS the poultry industry suffers major economical losses annually. Therefore, the poultry industry is in need for a way to control MAS, such that one or more of the assorted disease conditions observed in broilers can be prevented.
Reoviruses are ubiquitous in poultry world-wide. Reoviruses have been found to be the causative agent of an arthritic condition affecting the major weight bearing joint capsules and tendon sheets in the legs, designated as viral arthritis/tenosynovitis.
In some reports reoviruses were also isolated from chickens displaying MAS associated disease condition. In these reports it is speculated that the reoviruses have an etiological relationship with one or more of the MAS associated disease conditions, but firm proof of the involvement of reoviruses in MAS was not provided therein.
In van der Heide et al. (Avian Diseases 25, 847-856, 1981) a reovirus was isolated from the intestines of young broilers with clinical diarrhoea. Although this reovirus isolate was capable of inducing lesions of tenosynovitis and femoral head fractures and osteoporosis, this isolate did not consistently induce diarrhoea in chickens experimentally infected with the reovirus.
Page et al. (Avian Diseases 26, 618-624, 1982) isolated reoviruses from a flock experiencing lameness, stunting and erratic feather development. Although, the oral inoculation into susceptible broiler-type chickens produce a clear effect on weight gains, feather development and induced lesions in a number of organs, the induction of diarrhoea or wet letter was not reported.
Hieronymus et al. (Avian Diseases 27, 246-254, 1983) reported the isolation of several strains of reovirus from the intestines of chickens with suspected MAS and determined the antigenic relationship of these strains with reovirus strain S1133 which is commonly used as a vaccine strain for the control of infectious tenosynovitis. The authors confirm that despite the fact that the reoviruses were isolated from chickens with clinical MAS, it remained to be proven that the reoviruses were the causative agent of MAS.
Eidson et al. (Poultry Science 64, 2081-2086, 1985) investigated the effect of an inactivated reovirus vaccine, derived from the C08 strain isolated by Hieronymus et al., in broiler flocks experiencing problems with MAS as well as tenosynovitis. Although the vaccine had a positive effect on body weight of the broiler population, there was no difference in feed conversion observed. Furthermore, no effect of the vaccine on enteritis-associated disease conditions, such as wet litter was reported.
Rosenberger et al. (Avian Diseases 33, 535-544, 1989) also isolated several reovirus strains from the tendons and bone marrow of field-reared commercial broiler chickens. Although, chickens inoculated with the reovirus strains were examined for clinical disease, signs of diarrhoea or wet litter were not reported.
Kouwenhoven et al. (Avian Pathology 17, 879-892, 1988) also investigated the role of reovirus in the malabsorption syndrome. These authors could not reproduce MAS with reovirus isolated from a field case, and concluded that reovirus is not the primary etiological agent of MAS. It is speculated therein that infectious agents, including reoviruses and adenoviruses may act as some kind of trigger in the malabsorption syndrome.
However, in addition to the above-mentioned publications, many other viruses have been associated with MAS. These include rotaviruses, parvoviruses, enterovirus-like viruses, togarvirus-like viruses, coronavirus-like viruses, adenoviruses and caliciviruses. Additionally, it was also suggested that bacteria be involved in the aetiology. MAS-like field syndromes have also been attributed to mycotoxins in the prior art and it is speculated that mycotoxins or other toxins should not be ignored as the causative agent of MAS.
In a recently published review (World Poultry 14, 57-58, 1998), McNulty summarised the state of the art on MAS. McNulty stressed the non-availablity of a vaccine against MAC and stated that the virus isolations, as well as the virologic and microbiologic examinations of samples isolated from the field reported so far have not provided useful results with regard to the identification of the causative agent(s) of MAS. McNulty speculated that this approach is not likely to yield useful results. Instead, management measures in MAS affected production sites is considered to be the best weapon for control of MAS.
Hence, there still exists a need for a vaccine which induces an effective protection against certain enteric problems experienced by chickens, such as the enteric problems associated with MAS, resulting in the secretion of too liquid faeces and/or maldigested feed.
Moreover, avian reoviruses display considerable antigenic heterogeneity and the emerging of new antigenic classes of avian reoviruses may have important implications for the use of reovirus vaccines in poultry.